General methods used to protect data that is exchanged between two nodes are an IP security protocol (IPSEC) method and a transport layer security (TLS) method. The IPSEC method is essential for security of a control message in a mobile IP that is suggested by the Internet Engineering Task Force (IETF). An IPSEC is constructed with an Internet key exchange (IKE) for performing a key exchange function in a user area, an authentication header (AH) for protecting data in a kernel area, and an encapsulating security payload (ESP). Since a message is protected from an IP header, it is impossible to protect data for each service application or to apply quality of service (QoS). On the contrary, in the TLS method, it is possible to protect data for each application, and however, it is impossible to perform a security function for the IP header.
In order to protect data, the two nodes have to share the same key information. In order to share the same key between the two nodes, the IPSEC uses the IKE, and the TLS uses a key exchange function of its own. A mutual authentication process between the two nodes is essential to perform the key exchange function. A certificate is mainly used for this mutual authentication process. That is, a key is necessary for security of data. The certificate is needed so as to exchange the key. A security infrastructure such as a public key infrastructure is needed so as to manage the certificate. In addition, since the key exchange protocol has a considerably complex structure, it is difficult to apply the key exchange protocol to a wireless environment.